Sunday, 12 August 2012

Summary

This article provides accessible links and information on the fruit pest Spotted Wing Drosophila (SWD, Drosophila suzukii) , brought together from a variety of sources. The information is current as of August 2012. This is still prior to the likely landfall of Spotted Wing Drosophila (SWD) in the UK. It includes Fera UK advice plus experiences of growers and researchers in the US and Europe where SWD is already present. This information should be of interest for professional fruit growers and for gardeners wanting to know more on how to detect the pest and manage it when it does arrive in the UK.

Introduction

I learnt of a crueller reality for fruit growers in the UK when the owner of one visited my Cambridge Open Studio photography exhibition in July 2012. Fruit growers are looking out with dread for the first signs of the destructive fruit fly invader, Spotted Wing Drosophila (SWD), in the UK.

First recorded in Japan in the early 1910s, Spotted Wing Drosophila (SWD) was detected in the US and southern Europe by 2008. By November 2011 a specimen was found in Ostend, Belgium, just a Channel hop away (1).

It is only a matter of time before Spotted Wing Drosophila (SWD) invades the UK. Currently Spotted Wing Drosophila (SWD) is on the list of Notifiable Pests and Diseases set by Defra/Fera (2).

The bad news is countered by the upsurge on recent experience and publications in Europe and the USA. We can benefit from the experience gained by growers and researches who are already battling with Spotted Wing Drosophila (SWD, Drosophila suzukii).

Damage caused by the Spotted Wing Drosophila (SWD)

Within the first year of Spotted Wing Drosophila reaching California, it caused $500 million actual loss due to pest damage. The effect was very variable, with some areas having little or no loss and others up to 80% crop loss (3). Details for individual crops over three US counties are given in (4).

Why Spotted Wing Drosophila (SWD) has such an impact

The reasons for the potentially severe effects and fast spread of Spotted Wing Drosophila (SWD) include

2. Ability to attack ripening fruit

Female Spotted Wing Drosophila (SWD) target ripening fruit. They have a serrated ovipositor (egg-laying tube) with which they cut through the fruit skin and lay eggs inside. This is different to the familiar fruit flies, where eggs are laid on the surface of ripe fruit and the hatched larvae can only enter through any existing break in the fruit skin that they find (5)(6).

3. Exponential reproduction in a growing season

One to three eggs are laid per fruit. On average 400 eggs are laid during a lifetime that may range from 10 to 59 days. The hatched larvae feed mainly within the safety of the fruit and pupate. Adults can emerge from the fruit after a minimum of 8 days, depending on temperature. Up to 10 generations of Spotted Wing Drosophila (SWD) can be produced within a growing season (5)(6).

Each individual fruit can be infected by a number of different females, increasing the damage caused by the feeding larvae. Further causes of fruit damage are opportunistic infection by other fruit flies, fungi, yeasts and bacteria through the holes created (5)(6).

4. Wide temperature tolerance for breeding

Spotted Wing Drosophila (SWD) is tolerant of a wide range of temperatures, reproducing between 10 deg C and 30 deg C. Adults are cold tolerant and it is believed that mated females overwinter (5). The UK is well within its comfortable reproductive temperature range.

5. Long distance spread in fruit and also by flight

Long distance spread is likely to be via larvae and pupae hidden within harvested fruit moving across borders. Tracking studies on the close relative Drosophila melanogaster in 1961 showed that fruit flies could also travel at least 4.4 miles upwind within 24h (7). Targeted flight towards a food source by Spotted Wing Drosophila is quoted at 150m, they can however fly as far as 10km (8).

The Strategy: Monitor, Identify Control

With such a combination of factors creating a high potential for damage, how do you respond and control Spotted Wing Drosophila should it appear in your region?

The UK Food and Environment Research Agency (Fera), under Defra, put together a fact-sheet on Spotted Wing Drosophila (SWD) in 2010 (6) which included advice on potential control measures.
The pest has not entered the UK, so Fera also sent out a consultation invitation in January 2012 (11) requesting input on identifying the most appropriate response to SWD in the UK. Two options open for consideration were

No statutory action, leave industry to manage

Work in collaboration to limit spread, including limited statutory action when necessary

The document also outlines control options, indicating that an integrated approach will be needed (integrated pest management IPM).

The key message is to catch Spotted Wing Drosophila (SWD) as early as possible. The information sheet for Washington State wine grape growers gives the excellent mantraMonitor, Identify, Control
as an excellent memory aid (9).

Monitoring and identification can already be done by any grower in the UK with minimal effort.

Monitoring and Identifying Spotted Wing Drosophila

Fruit flies can be collected in 5% apple cider vinegar traps, also known as ACV traps (10)(11). These can easily be made economically from yoghurt pots or plastic bottles. Pierce the sides with holes a few millimetres wide or cover the opening with loose mesh cloth that will let fruit flies through. Add apple cider vinegar and a drop of detergent. Apple cider vinegar still appears to be the most effective attractant. The detergent serves to kill the flies. The pictures in the slideshow below show the results of an impromptu trial I conducted at home.

Traps should be set either as soon as temperatures exceed 10 degrees C or just before your crop is about to start ripening.

As a fruit grower, the simplest way to identify potential Spotted Wing Drosophila (SWD) from other fruit flies is – by the single wing spot on each of the wings. (The following photos are from specimens captured in Milton, Cambridgeshire, 2016)

Note: Only the males have the wing spots. As the sex ratio is lightly higher for females than males with this fruit fly early in the year, detecting males strongly suggests females are also present. Late Summer and Autumn, more males than females were found in traps in affected areas (12).

If you have access to a stereo-microscope, you can also search for females of SWD as they have a characteristic serrated (toothed) ovipositor (egg laying tube).

More pictures of male spotted wings and of female ovipositors to aid identification can be found in references (6)(8)(10). Holes in fruit created by egg-laying female Spotted Wing Drosophila are also another indicator of a potential problem.

Picture of D. suzukii by Wikimedia Commons contributor M Francisco

Control: Current Recommendations

The UK primary objectives (6, 10) are to:

Control the adult flies before they can lay eggs in the fruit and

Reduce fly populations available to re-infest later crops or carry over to the following year.

The following strategies can be used in an Integrated Pest Management for your crop if Spotted Wing Drosophila has been identified in your area.

Netting:

Japanese research in 2007, quoted in (5), showed that netting of blueberry orchards with a 0.98 mm mesh net completely prevented blueberry damage by Spotted Wing Drosophila.
Since 85% of commercial soft fruit is grown in poly-tunnels in the UK, growers may be able to include the fine netting as part of their poly-tunnel design.

More frequent harvesting/crop hygiene

More frequent harvesting of ripe fruit and removal of over-ripe, infected and leftover or spoilt fruit from the crop is important. It reduces the rate of fruit fly population growth during the growing season.

Spotted Wing Drosophila populations grow exponentially towards the end of Summer and into Autumn if unchecked. A larger surviving population of female fruit flies that can overwinter will create problems for you in the following year.

Some of the US advice is more controversial from an environmental perspective. The rationale proposed is that wild fruit and hedgerow plants will act as natural reservoirs of SWD and should therefore be removed close to agricultural areas (13)

Treatment of infected waste

Do not simply compost infected material as this does not kill larvae in the fruit and could actually make the situation worse. Fera mentions the use of deep burial (11). Solarization, insecticide treatment, disposal in closed containers, crushing, cold treatment, bagging and burial are all being investigated (6, 10, 14).

Bait traps

There are mixed messages about the effectiveness of bait traps for mass trapping of Spotted Wing Drosophila (mentioned in 11). The Penn State Extension quotes Japanese work where trapping with 60 to 100 vinegar traps per acre decreased SWD numbers (13).

Organic control

As an organic grower you would already include the above control measures in your organic IPM. In addition, you may be able to use approved pesticides for organic growers.

Organic growers in Utah and the Pacific Northwest have used a rotation between Entrust, a spinosad pesticide and Pyganic, an organic pyrethrum insecticide (10, 15). Preliminary field trialling results with strawberries and caneberries using Entrust and Pyganic gave up to 5 days of Spotted Wing Drosophila control (16).

A Grower in the UK you need to check whether these organic pesticides can be used on your crops. We need more alternatives as a limited selection of approved pesticides will favour resistance developing.

Chemical control

Pesticides are applied as foliar sprays. The recommended timing is about two weeks before harvest, whilst the fruit is ripening. The aim is to try to kill the adults before they lay eggs. If your monitoring identifies high levels of Spotted Wing Drosophila, you can spray earlier.

Fera in the UK recommends using insecticides with generally lower human toxicity – such as spinosad, imidacloprid, acetamiprid and certain pyrethroids that have been shown to be effective. Obviously you will need to use the products registered for your individual crops (6).

The fast generation time of Spotted Wing Drosophila promotes the development of resistance if you use just one pesticide throughout a season. Delay the onset of resistance by rotating different classes of insecticides during your season.

Detailed lists of pesticides permitted in the US and recommended for their crops are given in (5, 13, 15). A comparison of insecticides looked at their impact on Spotted Wing Drosophila both in the laboratory and the field (17). Pyrethoids, organophosphates and spinosyns provided 5 to 15 days of residual control. In contrast, neonicotinoids were not so effective in killing adult flies and are currently not recommended.

IPM Area

Spotted Wing Drosophila can survive in wild fruits and travel large distances. Therefore integrated pest management needs to be applied to a wider area. This means taking in to account treatment in neighbouring crops and how to control SWD numbers in natural areas without detrimentally affecting pollinators and wildlife.

Future Control Options

The advance of Spotted Wing Drosophila in Europe and the US has generated a flurry of research into control measures (5, 18). Based on experience with close fruit fly relatives and other successful pest programs, a whole range of different options are in the pipeline.

Chemigation

Pesticide sprays may not penetrate deep into the foliage. The alternative could be application in a mist spray. In the US, trials were conducted on Blueberries using “chemigation”; they applied the pesticides with the cooling mists for the crop during the hottest part of the season. SWD mortality was well above 90% with mists created using Netafilm micro-sprinklers (19).

Bait trapping

Fera and others mention the possibility of bait trapping (5, 6). Pesticides are added to bait that contains attractants. Feeding adults are then killed. Whilst baits for some fruit flies are known, the best bait for Spotted Wing Drosophila is still to be determined. Currently 5% apple cider vinegar still seems consistently effective as an attractant.

Parasitic wasps

Parasitic wasps have been used to control aphids in greenhouses and poly-tunnels. Naturally occurring parasitic wasps have either been hatched from Spotted Wing Drosophila pupae in the states (13, 20) or tested on SWD in Europe (21). In the latter, only parasitic wasps that attacked the pupae of Spotted Wing Drosophila could kill the pest, namely Trichopria cf drosophilae and P. vindemmieae. It appears that the larvae have some resistance to these wasps. Work is still continuing and the best control wasps then need to be multiplied to commercial levels.

Longer term options

Cini and coworkers wrote their review after the Trento international meeting on Spotted Wing Drosophila. This was attended by about 180 people (5) and also included ideas that would require more research and development.
Ideas being pursued are:

Searching for more organic and chemical control agents. We need more control agents in the pipeline. Resistance to existing agents will inevitably arise over time.

Mating disruption using sterile male Spotted Wind Drosophila.

Finding and producing Spotted Wing Drosophila pheromones (sex-attractants) that can be used in traps.

Use of Drosophila specific DNA viruses.

Exploiting the ability of bacteria from the genus Wolbachia to infect more than half of living insect species. Wolbachia can cause male feminisation and even death.

One of the accelerating factors in any research into Spotted Wing Drosophila is its relative Drosophila melanogaster, This fruit fly has been the work horse of genetics research for over a century. D. melanogaster DNA has been sequenced for some time. On the 4th July 2012, the Fondazione Edmund Mach announced that they had sequenced the Spotted Wing Drosophila (Drosophila suzukii) genome (22). These factors raise the hope that findings from one model system can be rapidly transferred to Spotted Wing Drosophila.

Summary

This article was written before the likely landfall of Spotted Wing Drosophila (SWD, D. suzukii) in the UK. It includes UK advice and also the experiences of growers and researchers in the US and Europe where SWD is already present.

Spotted Wing Drosophila is a notifiable pest. The article describes how to monitor for SWD using 5% apple cider vinegar traps and how to identify the male and female Spotted Wing Drosophila if caught.

Fera has issued a Plant Pest Factsheet for Spotted wing drosophila Drosophila suzukii (6) which is the official source of information on the pest and its control in the UK. There is currently a consultation on UK policy towards Spotted Wing Drosophila (11) which should provide further official guidance when published.

This article provides information both on the existing UK guidance and the wider experience of growers in the USA and Europe who are currently battling with Spotted Wing Drosophila. This report should therefore be of interest for professional fruit growers and for gardeners in the UK who expect the arrival of the pest and are looking for a wider overview.

About the Author

Chris Thomas, PhD, is a former scientist with 20 years of experience in plant molecular biology and plant pathology.

I am now director of my own company, Milton Contact Ltd (www.miltoncontact.co.uk) founded in 2004. The company is active in a totally different area – helping companies communicate with their clients in print, pictures and person. Within the UK this involves anything from photography, writing articles and through to editing, designing and publishing books for local authors. Internationally, I assist overseas companies interested in entering the UK market.

This article was an opportunity I could not miss. It was a pleasure to be able to combine:

A continued interest in plant science and pathology

Communicating the information to an audience of farmers, growers and gardeners

The ability to publish it and make it available to all.

My thanks to:

The fruit grower who came to my Cambridge Open Studios exhibition, saw the photo of a fruit fly head and first made me aware of the existence and risk of Spotted Wing Drosophila entering the UK
Thanks also to my neighbour Jo who had red wine vinegar, apple cider vinegar and convenient plastic bottles on hand when she heard of my interest in setting up some fruit fly traps

(ADDED JAN 2013. Some sites recommend concentrated yeast extract and sugar solutions. I personally would not recommend using these for the following health and safety reason: Similar solutions are used for bacterial cultures, including fecal bacteria. Incidental contamination by animal or bird droppings, or flies that feed on droppings could result in a very high bacterial growth that may be detrimental to you. Also, it is very hard to see the SWD or any fruit flies in a fermented yeast soup!)

UPDATE Jan 2013. SWD is now NOT notifiable to Fera, because of its biology and it being almost impossible to control at present. Please DO send samples of any trapped adult Drosophila flies to: Dr Michelle Fountain, East Malling Research, New Road, East Malling, Kent ME 19 6BJ.

2 comments:

Fera are working with industry organisations (such as HDC and NFU) to raise awareness and provide advice to growers, to help prepare for its arrival. You may want to keep track of plant health developments through Fera’s Plant Health What’s New Page at http://www.fera.defra.gov.uk/plants/plantHealth/ as this is where they would report any significant developments.

Spotted wing drosophila (SWD) was found and confirmed at East Malling Research (EMR) in Kent Early September, 2012.

If you suspect SWD is present on your farm, you should send samples of any trapped adult Drosophila flies to: Dr Michelle Fountain, East Malling Research, New Road, East Malling, Kent ME 19 6BJ.

Dead specimens should be sent in a crush proof, leak proof container. Fresh caught insects should be placed in a small tube or similar container containing a couple of drops of apple cider vinegar. Sample bottles supplied by a chemist are adequate if nothing else can be found.

Please include contact details along with information on the typle of crop and location found. This will allow East Malling Research to track where the pest has spread to.